Rhizobia are the aerobic bacteria that are present freely in the soil and in the root region of both leguminous and non-leguminous plants. These bacteria helps in nitrogen fixation. Hence, they are called nitrogen fixing bacteria. They live in symbiotic association with roots of leguminous plants. They convert the atmospheric nitrogen into absorbable forms such as nitrate and nitrite so that plants can take them up from the soil. Hence, they contribute a lot in regulation of nitrogen cycle in our environment.